1. Field of the Invention
This invention relates to a method and apparatus for operating the nuclear reactor in an electric power generation system when 100% power can no longer be maintained at equilibrium. Specifically, it is directed to suppression of reactor xenon poisoning near the end of the fuel cycle by cycling of the load imposed on the reactor in such a manner that 100% power can be maintained at least during part of the day.
2. Description of the Prior Art
In the very early days of the nuclear power industry it was envisioned that fuel costs would be so low as to be an almost insignicant part of the cost of generating electricity. With the recent sharp increases in the cost of uranium together with the steady rise in enrichment price this is no longer the case. We have now reached the stage where the cost of the fuel can contribute up to 30% of the cost of producing electricity. Under these conditions every effort must be made to reduce fuel costs.
The cost of fuel can be broken down into three components: (1) fabrication price, (2) ore price, and (3) cost of enrichment services. The fabrication price has steadily declined over a number of years under the influence of competitive pressure. There is little likelihood of further substantial reductions. On the other hand, both the ore price and the enrichment price continue to increase and the only prospects for savings are to reduce the quantities of ore and enrichment which must be used. Reducing ore and/or enrichment necessarily reduces the initial fissile inventory of the core and hence the initial reactivity. Hence these savings can only be effected if a compensating reactivity gain can be found.
It is well known that xenon formed during the operation of a nuclear reactor adversely affects reactor reactivity. Xenon 135 is an isotope with a very, very high cross section for thermal neutron absorption; it is mainly formed by decay of Iodine 135 which is itself formed directly by fission. Xenon 135 will itself decay to a non-absorbing daughter unless a neutron is absorbed within a few hours. None of the stable or long-life members of this chain have any significant commercial value, and Xenon 135 is simply a reactivity burden for a fixed-fuel thermal reactor such as a pressurized water reactor (PWR).
The burden imposed on a nuclear reactor by xenon poisoning is very great. If the effects of xenon poisoning could be fully suppressed, a typical fuel cycle would run three months longer for the same fuel enrichment; or the same fuel cycle length could be attained with the enrichment reduced by 0.4%. In the latter case, a saving of over $5.5 million a year could be realized for a typical PWR. The reality of the situation is, however, that under present practice, sufficient excess reactivity must be provided to compensate for equilibrium xenon poisoning in order to maintain a reactor at full power over the entire length of the fuel cycle. In a PWR this excess reactivity amounts to about 3% in addition to that required to compensate for burnup effects. Further, if the reactor power is reduced, the xenon poisoning will initially increase over a period of about six hours before starting to decay. This xenon transient is generally regarded as an operating constraint as it limits the ability of the plant to return to full power. However, we have found that, by careful planning, it can be turned to advantage and used to offset some of the equilibrium xenon poisoning.
In the typical electric power generation system, the load carried by the system can vary considerably over a 24-hour period. To accommodate these variations, some of the generators are run continuously at full power; others are throttled to accommodate the variations in load; and still others may be started up and placed on line only to handle peak loads. The first group of generators are said to be base loaded. Because of the relatively cheap fuel costs compared to fossil fuels, and the high capital investment, nuclear power plants are usually base loaded. The base loaded nuclear plants are run until full power can no longer be maintained. They are then shut down for refueling or, in some instances, the running period is extended by a coast-down mode of operation in which the maximum power output is slowly reduced until a desired burnup limit is reached. In systems where nuclear plants supply more power than needed for base load, some nuclear plants in the system are used in load sharing and frequency participation; however, in such situations, load is varied to meet system requirements and xenon poisoning is still a constraint near the end of the fuel cycle.
It is a primary object of the present invention to reduce the cost of nuclear fuel required in a nuclear electric power generating plant.
It is also an object of the invention to achieve the primary object by suppressing the effects of xenon poisoning on reactor reactivity.
It is another object of the invention to achieve the previous objects by varying the loading on the reactor after 100% power can no longer be maintained at equilibrium.
It is yet another object of the invention to achieve the previous objects while maintaining 100% power during the major portion of the daily operating cycle.
It is an additional object of the invention to achieve the previous objects by operating the reactor at reduced power during part of the day and by reducing the reduced power level on successive days.
It is still another object of the invention to reduce the power level during the reduced power periods only to the maximum reduced power required to be able to operate at 100% during the high power level period.
It is also an object of the invention to achieve the previous objects by periodically predicting the maximum reduced power level as a function of present reactor flux and a predetermined daily loading schedule.
Other objects will become evident from a reading of the summary of the invention and detailed description of the preferred embodiment of the invention, which follow.